In prior thermophotovoltaic (TPV) circuit designs, GaSb TPV cells are soldered down onto metal pads on a copper substrate where a thin insulating layer isolates the metal pads from the copper substrate. Flexible interconnects are then bonded from pads on the top of the cell over to pads on the circuit. Finally, mirrors are attached over the lead bond areas. Since the leads have to carry large currents in TPV circuits, the lead area is fairly large. Consequently, the mirrors typically cover 40% of the circuit area.
There are two disadvantages to this configuration. First, although the lead attach step can be automated, it is still quite time consuming. Eliminating the lead bonding step is desirable. Second, the mirror area leads to a loss in system efficiency. This system efficiency loss comes about as follows. If radiation heat transfer were the only heat transfer mechanism, then the mirror area would not have a negative impact because the radiation hitting the mirrors is simply returned to the infrared (IR) emitter in a TPV system. However, heat transfer also occurs by conduction and convection through the air space between the IR emitter and the cell circuit. Recent measurements indicate that this heat transfer process is as much as 30% of the total heat transfer rate. The mirror area is additional area that does not contribute to electric power production but does receive heat through the air heat transfer mechanisms. It is therefore desirable to reduce the mirror area and to increase the fractional active cell area.
An alternative circuit concept for solar photovoltaic application is to shingle the cells, with the top of one cell attached to the underside of the next cell. Unfortunately, this idea has been tried without success. The problem is that during thermal cycling, the substrate material expands at a different rate than the cell so that the rigid bond joint eventually is pulled apart forming an open circuit. A need exists for flexible leads that avoid this failure mechanism.